The long-term goals of this project is to evaluate procedures that will improve radioimmunotherapy (RAIT) by either increasing tumor accretion or decreasing normal tissue uptake, i.e., increase the therapeutic window. Preclinical testing has shown that the second antibody (SA) procedure reduces concentrations of radiolabeled antibody in the blood, increases tumor/nontumor ratios, and improves the therapeutic ability of 121I-labeled anti-CEA antibody. A humanized anti-CEA antibody (hMN-14) and an anti-idiotype antibody (hW12) were prepared, and clinical testing is planned to test the hypothesis that this procedure will permit a substantial increase in the maximum tolerated dose (MTD) in patients, and provide higher radiation doses to tumors. The clinical trial first will determine the optimal protein dose for humanized W12, as well as a galactose-conjugate of this antibody, which is expected to accelerate W12 clearance from the blood. Finally, a Phase I RAIT trial will be performed to assess the MTD with SA. Preclinical studies will continue to evaluate the avidin (or streptavidin) biotin pretargeting methodology (A-B method). The hypotheses behind using pretargeting approaches over directly radiolabeled antibodies is that they will provide an alternative approach for obtaining high tumor uptake with minimal normal tissue retention. Pretargeting approaches also enable the use of 90Y-labeled effectors, whereas the SA procedure is restricted to therapy with 131I-labeled Mab. Previous progress with the streptavidin-MN-14, 111In-DOTA-pep-biotin approach has yielded extraordinarily high tumor/blood ratios with good tumor/nontumor tissue ratios. Thus, there is optimism that this procedure may be adapted for therapeutic uses. In this regard, a clinical trial with this approach, or a modification thereof, is expected. In addition, new variations in the A-B method will be evaluated including: "3-step" targeting, fractionation of effector dosage, combination of 131I-MN-14 with pretargeting, modifications in the effector to increase residence time in the blood to increase its uptake in tumor, and procedures to increase the specific activity of either the effector or the primary antibody. All of these variations in A-B targeting will be evaluated in nude mice bearing human colonic tumor xenografts, along with a comparison to directly radiolabeled antibody with or without SA.